Accretion definition, an increase by natural growth or by gradual external addition; growth in size or extent. See more.
Alveoli are tiny sacs within our lungs that allow oxygen and carbon dioxide to move between the lungs and bloodstream. Learn more about how they function and quiz your knowledge at the end.
Deep within the lungs exists its constituent alveoli sacs, the sites responsible for the uptake of oxygen and excretion of carbon dioxide. There are three major alveolar cell types in the alveolar wall (pneumocytes): Type I pneumocyte (Squamous Alveolar) cells that form the structure of an alveolar wall. 2  Type II pneumocyte (Great Alveolar) cells that secrete pulmonary surfactant to lower the surface tension of water and allows the membrane to separate, thereby increasing the capability to exchange gases.